Establishing Iterators

[phorward]

This PR implements iterators for Tokay.

• RefValueIter is a trait that implements any iterator
• Iter is the Tokay iterator value object
• MethodIter is an iterator working on indexed method calls
• RangeIter is an iterator for counting integer values
• MapIter is an iterator with a mapping an iterator to a callable, which also can be used as a filter
• EnumIter is an iterator mapping an iterator with an index

The following Tokay functions are made available:

• iter() creates an iterator from an object, by calling object.iter() or automatically a MethodIter on an object
• iter_rev() reverses an iterator, if possible
• iter_next() returns the iterators next element
• iter_collect() collects and iterator into a list; is used by list() on an iter as well
• iter_enum() creates an enumerated iterator
• iter_map() creates a mapped/filtered iterator
• range() creates a range counting iterator

The implementation is only a place to start. There are still some restrictions and problems discussed in PR #101.
There's also the need for aggregation functions as described in issue #88.

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What's already working:

for i in (1,2,3) print(i)

d = (first => 1, second => 2, third => 3)
for k in d print(k)
for k in d.keys print(k)
for k in d.items print(k)

 # create lists from iters
print(list(iter(d)))
print(list(d.keys))

Still existing problems:

• Efficiency?
• range() function likewise Python works without a target object, just on a min-max-step construct
• How to implement dict.keys() or dict.items() efficiently?
• What about custom iterator behaviors, as one cannot inherit from iter; Tokay is NOT an object-oriented language.
• What about reversable iterators?
• Iterator-Functions
  • len() - counts the iterator
  • map() - maps the iterator to a function; using void paradigm acts like a filter.
  • enum() - returns a list of [index, item] for each entry
  • collect() - collect iterator into a list
  • min(), max(), sum(), avg() - aggregate functions on iterators (will be separately done with Provide iterator aggregation functions min(), max(), sum(), avg() #88)

[phorward]

Ok I think Iterators are now in a state to be merged into Tokay.

[phorward]

Ok I think Iterators are now in a state to be merged into Tokay.

After some consideration, let's finish iterators by providing some more functionality.

[phorward]

#88 refers to this

[phorward]

The iterator feature in the currently started version with the mapping unfortunately seems to be a bottomless pit, since several problems arise:

1. due to the possibility of using functions as callbacks, the context must necessarily be passed and passed through, which is partially implemented here.
2. since the iterator is borrowed mutable a further access possibly within a callback to the same iterator object leads inevitably to a panic.
3. in general the passing of the context/args/nargs parameter configuration (or even if parameters are already on the stack) should be done in the form of a separate struct or enum, but this entails major rebuilding work in the entire code.

It would therefore be better if the map feature - possibly even using a yield function - were implemented in pure VM code.

Nevertheless, this branch contains important and necessary changes, which is why it should be remembered. Whether the map feature will be removed again or not remains open at the moment.